The MRI has become an invaluable tool for imaging and exploring the internal anatomy without surgery. MRI has the ability to distinguish healthy from diseased tissue, fat and muscle and adjacent structures within the body which other modalities cannot differentiate. MRI uses safe radio waves and a strong magnetic field to generate the information, which is processed by a computer, to create an image. There are now about 10,000 MRI machines currently in use in the United States.
Because of the desire for higher quality and higher resolution, the static magnetic field strength has increased steadily over the last 25 years, from a fraction of a Tesla to the current situation where 3 Tesla magnets are common and 7 Tesla magnets are now appearing on the market. Most new magnets for MRI are superconducting and as a result very expensive to start up, causing the requirement for the magnet to be kept always on. In an emergency, these superconducting MRI magnets cannot be turned off quickly.
The strong magnetic field associated with the area near to the MRI presents a danger of ferromagnetic objects being attracted to the strong magnetic field. There are numerous recorded incidents of objects becoming flying projectiles resulting in injury to the nearby personnel or the patient. Costly damage may be caused to the MRI machine itself from the impact of the projectile.
In order to reduce the risk of a ferromagnetic projectile accident, a ferromagnetic detection system as described in U.S. Pat. No. 7,489,128 MRI PROTECTOR is employed at the entrance of the room containing the MRI machine. This device will provide an alarm should a device containing ferromagnetic material pass through the ferromagnetic detector portal.
It is sometimes necessary to bring devices which contain ferromagnetic material into the room containing the MRI machine. As a result, ferromagnetic detectors do not prove useful in this situation other than to alert personnel that a potential risk may be present. Medical equipment, such as infusion pumps, contrast injectors, patient monitors as well as anesthesia machines, is commonly necessary in the room containing the MRI machine.
In addition to the potential projectile risk posed by medical equipment brought into the magnetic field of the MRI, many medical devices do not function correctly or are damaged when operated in high magnetic fields. A contrast injector, for example, may inject an incorrect quantity of contrast agent when the device is exposed to a magnetic field greater than its design magnetic field maximum value.
A variety of medical devices can operate safely in the vicinity of the MRI magnet if precautions are made to insure that the medical device is kept in an area where the magnetic field strength is below the limit where the device will perform correctly and/or will not present a ferromagnetic projectile risk. Medical equipment brought into the MRI room is quite commonly on carts with caster wheels. The equipment may, therefore, be easily moved or accidently bumped into an area where the magnetic field is dangerously high. Rarely are there markings indicating the various magnetic field strengths on the floor or elsewhere. As a result, the personnel may not be aware of the present danger. What is needed is a device to alert the personnel of the dangerous magnetic field in which the medical device is immersed.
The electromagnetic signal produced during MRI and used to create the image is very weak. For this reason, modern MRIs are contained in a specially constructed electromagnetically shielded room, sometimes called the screen room or MRI room. Any piece of electrical or electronic equipment operating inside this screen room can be a potential source of interference. Electromagnetic interference can cause a degradation of the diagnostic quality of the image produced by the MRI process. As a consequence, electrical equipment is not brought into the screen room unless absolutely necessary and only then after a rigorous testing process to validate that the device does not degrade the performance of the MRI. Even after this validation process, there is still a significant risk that the MRI image quality may be compromised.
Equipment such as patient monitors, infusion pumps and power injectors are moved frequently within the screen room. As such, a power cord supplying electricity to the equipment is inconvenient as well as being a potential risk of electromagnetic interference. Because of this, most electrical equipment brought into the MRI screen room is battery operated.
A variety of gaussmeters are on the market utilizing various sensing techniques including Hall Effect, flux gate and anisotropic magneto-resistive. Magnetic field strength alarms are known for detecting the magnetic field strength in the MRI environment. For example U.S. Pat. No. 4,954,812 entitled MAGNETIC FIELD ALARM INDICATOR discloses a magnetic field alarm detector comprised of a plurality of magnetic field detectors mounted in a horizontal plane for detecting respective rotational components of a magnetic field. To achieve an even moderately uniform sensitivity in a 360 degree field of view, at least 3 sensors must be used. More sensors are needed for better rotational sensitivity uniformity. The Hall Effect sensors used as well as the necessary processing electronics use a significant amount of electrical power. To be effective, the invention disclosed consumes power anytime the device described is monitoring the magnetic field strength. As a consequence, the batteries must be charged or replaced regularly. Further, since the disclosed invention must be powered at all times while the device is in the screen room, there is a significant risk of the device causing electromagnetic interference. This risk is increased if a number of pieces of medical equipment in the screen room are fitted with the alarm described in the invention.
U.S. Patent Application No. 2007/0132581 entitled METHOD AND APPARATUS FOR FERROUS OBJECT AND/OR MAGNETIC FIELD DETECTION FOR MRI SAFETY The device described attempts to address the power consumption issue by using an radio-frequency identification (RFID) system to turn the device on when it enters the MRI room and off again when it leaves the MRI room. This approach has potential of lowering the power consumption of the device under certain circumstances. However, the types of medical equipment described above are quite commonly stored in the MRI room. As a result, there is no power consumption reduction in this common situation since the device must be left powered. Further, the risk of electromagnetic interference with the MRI machine is still present since the device is powered while it is in the MRI room. The risk of electromagnetic interference may actually be increased by the presence of RFID circuitry.